added format "image/svg+xml" to msSLDParseExternalGraphic - added thanks to Regione Toscana - SITA

added format "image/svg+xml" to msSLDParseExternalGraphic - added thanks to Regione Toscana - SITA

in this way it's possibile to add external svg URL in a SLD.

patch is useful only if mapserver is compiled with CURL